1. Field of the Invention
The invention relates to an inclination sensor with a level that has a liquid and a gas bubble under a cover glass in a housing, wherein a light source is arranged above the cover glass.
2. Description of the Background Art
Levels are customarily used for horizontal or vertical orientation of apparatuses such as measurement instruments or in construction, for example. These levels usually take the form of tube-type levels for one-dimensional orientation, or circular levels for two-dimensional orientation. The level is attached to the apparatus such that its gas bubble is located at a defined location in the level when the apparatus is in the oriented state. The oriented state can be established or reestablished with little effort using the level. The oriented state of the apparatus need not necessarily be horizontal or vertical. In principle, any desired angle of inclination can be specified for the oriented state by an appropriately tilted arrangement of the level on the apparatus.
When the gas bubble is in the defined location, it is referred to for the purposes of the invention as being centered.
In order to achieve a stable and reproducible position of the bubble as a function of the level's angle of tilt, in the prior art the interface between the cover glass and liquid is manufactured with a concave contour. The sensitivity of the level then depends primarily on the radius of curvature of the cover glass.
Arrangements are already known in the conventional art with which levels can be read electronically. Photoelectric principles are generally employed for this purpose. The majority of conventional art arrangements are based on transilluminating a level, wherein a light source and a photodetector are arranged on opposite sides of the level, in particular on the top and bottom.
For example, an inclination sensor containing a conventional tube-type level is described in DE 39 38 848 A1 whose housing consists of a light-transmitting material and whose interior is filled with a liquid surrounding a gas bubble. Located above the tube-type level is a light source that illuminates the entire length of the level. The light emerging from the light source passes through the transparent housing and through the likewise light-transmitting liquid and also through the gas bubble. A nonuniform light distribution is produced on the side of the level opposite the light source, and is analyzed by a photodetector located thereunder. Such devices are expensive, require a large amount of space, and generally require specially designed levels. Major disadvantages include the small difference in brightness of the light spot that is to be analyzed, resulting from the low difference in transmission of the gas bubble and liquid, as well as the washed-out contours of the light spot resulting from reflection and scattering effects at the optical interfaces. Sensing of the position of the light spot is thus subject to considerable uncertainties, and permits only small bubble excursions for unambiguous interpretation of the photocurrent signals.
Known from DE 36 34 244 A1 is an optoelectronic inclination sensor with a radiation emitter and a radiation detector, in which the radiation emitter and radiation detector are located above a light-reflecting medium, wherein the inclination is detected by reflection at a light-reflecting liquid, preferably at a mercury surface. The liquid is enclosed in a capsule that is only partially filled by the liquid. Detection of the position of a gas bubble is not possible with this arrangement.
Moreover, there is known from JP 10 227 635 A an inclination sensor in which a light source and a photodetector are arranged on the same side of a level, namely above the cover glass. This inclination sensor utilizes the principle of total internal reflection. When the level's orientation is horizontal, which is to say when the gas bubble is centered as defined for the purposes of the invention, the light from the light source incident on the gas bubble is totally internally reflected toward the photodetector at the interface between the optically denser cover glass and the optically less dense gas bubble. The light that is totally internally reflected in the centered case can be detected by means of the photodetector. If the gas bubble is off-center, no total internal reflection takes place, and thus no light incidence on the photodetector. It is thus possible to determine whether or not the gas bubble is centered using the detectable light. This serves as a binary indication of a horizontal, vertical, or other predefined orientation of an apparatus. This inclination sensor has the disadvantage that the direction and amount of inclination cannot be determined. Automatic orientation of an apparatus equipped with such an inclination sensor is not possible. Moreover, it is disadvantageous that the proportion of total internal reflection is only effective at adequately long distances between the light emitter and detector, which results in large, and hence costly, sensors. Depending on the dimensions of the level and the desired dynamic range (tilt angle), the use of total internal reflection at the contact surface between bubble and glass can even become ineffective.